In amusement facilities and others, a digital image is projected on a large-sized screen and dome-type screen in recent years: One of the known display devices for implementing this operation is a display device using a display element such as a liquid crystal element and digital micro-device, instead of the display device for projecting a film on a screen. This type of display device is equipped with a display element for outputting each of the R, G and B, for example, a prism for color-superimposition of the image outputted from each display element, a projection optical system for enlarging and projecting an image color-superimposed by the prism, and a screen for displaying the enlarged and projected image.
The display element to be adopted in the display device of this type is getting more and more highly intergraded every year, and the entire size of the element is getting larger and larger, accordingly. Increased size of the display element requires the prism size to be increased. To ensure the space for accommodating a large-sized prism, a projection optical system having a very long back focus is essential. Especially when color synthesis is to be made by a color synthesis prism, the color synthesis prism must be made larger (longer) if the display element is increased in size. To ensure a space for accommodating a large-sized color synthesis prism, a very long back focus is necessary.
In recent years, to increase the brightness of the image projected on the screen, there has been a growing tendency to increase the size of the illumination system. This requires a projection optical system having a very long back focus.
In recent years, the image outputted from the display element is required to be projected onto a large screen at a short distance. To achieve this objective, the projection optical system is required to have a wider angle (shorter focusing).
The close-packed structure of a display element in recent years requires a high-resolution projection optical system of reduced chromatic aberration. To put it another way, the close-packed structure of a display element requires the projection optical system to provide higher projecting performances. Particularly, if the chromatic aberration is not reduced to a low level, a high-performance and hence a clearly projected image cannot be obtained.
Projection optical systems related to the present invention are disclosed in the following Patent Literatures. Patent Literature 1 discloses a fisheye lens projection optical system for projecting the image of a film on a dome-shaped screen. Patent Literature 2 discloses a fisheye lens projection optical system for a liquid crystal projector. Patent Literature 3 discloses a fisheye lens projection optical system wherein the back focus of a certain length is ensured.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. Hei 4(1992)-69611
Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2005-221955
Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2005-128286